1. Field of the Invention
The present invention relates to an exposure apparatus and method used in lithography processes to produce, for example, semiconductor devices, liquid-crystal display devices, imaging devices (e.g., CCDS), or thin-film magnetic heads. The present invention is applicable to one-shot exposure type projection exposure apparatuses and methods, but particularly suitably applied to step-and-scan or other scanning exposure type exposure apparatuses and methods, in which a mask and a photosensitive substrate are scanned synchronously with a part of a pattern image being projected onto the substrate, thereby photolithographically transferring the mask pattern onto the substrate.
2. Related Background Art
In photolithography processes for producing semiconductor devices and so forth, one-shot exposure type projection exposure apparatuses, i.e., steppers, have mainly been used, in which a pattern on a reticle (or a photomask or the like) is transferred by exposure onto each shot area on a wafer (or a glass plate) through a projection optical system in a stationary state. Recently, however, attention has been paid to scanning exposure type projection exposure apparatuses, i.e., slit scan type, step-and-scan type, etc., in which a reticle and a wafer are scanned relative to a projection optical system, thereby sequentially transferring a pattern formed on the reticle onto each shot area on the wafer, in order to comply with the demand that a pattern of wider area should be exposed with the image-formation characteristics maintained at the desired level. It is possible according to the scanning exposure type of projection exposure apparatus to use approximately the largest diameter portion of an effective exposure field of the projection optical system by illuminating the reticle with light in the form of a slit, by way of example. Moreover, by synchronously scanning the reticle and the wafer, the exposure field can be enlarged in the scanning direction without being restricted by the optical system. Furthermore, because only a part of the effective exposure field of the projection optical system is used, the required accuracies for illuminance uniformity, distortion, etc. can be readily obtained.
Positioning accuracy is one of important performances of the above-described projection exposure apparatuses. More specifically, a semiconductor device, for example, is formed by stacking a multiplicity of layers of different circuit patterns on a wafer. Therefore, a circuit pattern drawn on a reticle must be precisely overlaid on a pattern already formed on each shot area on a wafer. It is necessary in order to obtain a high overlay accuracy to use a stage mechanism for accurately positioning each of the reticle and wafer. The stage mechanism comprises a reticle stage for mounting a reticle, a wafer stage for mounting a wafer, drive mechanisms for driving the two stages, respectively, stage control systems for controlling the drive mechanisms, respectively, and a main control system for controlling the two stage control systems overall. In operation, the positions of the reticle and wafer stages are accurately measured, and the corresponding stage control systems are operated on the basis of the result of the measurement, thereby precisely aligning the reticle and the wafer. Accordingly, devices for precisely measuring the positions of the reticle and wafer stages also play an important part in the alignment.
Usually, laser interferometers are used to measure the positions of the reticle and wafer stages. In an apparatus that requires ultra-precise positioning, i.e., a projection exposure apparatus, when the wavelength of a laser beam used by a laser interferometer (i.e., laser wavelength) has changed owing to a variation in the refractive index of the air, a correction for the laser wavelength change must be made. If such a correction is not made, the positioning accuracy degrades. More specifically, the detecting part of a laser interferometer generally outputs one counting pulse (up-down pulse) every time an object of measurement moves through a distance .lambda./N, where .lambda. is the laser wavelength, and N is an integer, e.g., 32, 64, 50, etc. Therefore, the travel distance of the object is obtained by multiplying the summation of counting pulses by .lambda./N. Accordingly, if an incorrect value is used as the laser wavelength .lambda., it becomes a measurement error as it is. Therefore, the conventional practice is to install, in the apparatus, sensors for measuring an atmospheric pressure, temperature and humidity, respectively (hereinafter referred to as generically "environmental sensors"), and to calculate a refractive index of the air on the basis of measured values from these environmental sensors, and further to correct the laser wavelength at the rate of once a unit time of about 2 minutes under normal circumstances. To correct the laser wavelength, mainly the following two methods have heretofore been used.
According to a first method, as shown in FIG. 4A, a special-purpose controller for wavelength correction is provided in order to correct the laser wavelength. Referring to FIG. 4A, the laser wavelength is corrected by a special-purpose controller 51 whether the apparatus is of one-shot exposure type or scanning exposure type, and the result of the laser wavelength correction is supplied to a stage controller (aggregate of a main control system and a stage control system) 52. The stage controller 52 corrects measured data from a laser interferometer on the basis of the result of the laser wavelength correction. According to a second method, the laser wavelength is corrected by the stage controller itself without providing a special-purpose controller for laser wavelength correction.
FIG. 4B is a flowchart for explaining an example of a method of correcting the laser wavelength by the stage controller itself. The flowchart shows an example of an exposure process in which n-number of shot areas are exposed in a scanning exposure type projection exposure apparatus. As shown in FIG. 4B, first a variable i indicating a shot area number is set to 1 at step 301. Then, environmental data, i.e., atmospheric pressure, temperature, humidity, etc., is read out from environmental sensors at step 302, and at step 303, refractive index data necessary for laser wavelength correction is set at step 303. Next, at step 304, a variation of the laser wavelength due to a change in the environmental data is calculated on the basis of the set refractive index data, and the laser wavelength is corrected on the basis of the result of the calculation. In this case, the positions of the reticle and wafer have already been measured with the respective laser interferometers.
At the subsequent step 305, the reticle stage is positioned to a scanning start position on the basis of the corrected laser wavelength, and the i-th (1st, in this case) shot area is positioned to a scanning start position through the wafer stage. Next, environmental data, i.e., atmospheric pressure, temperature, humidity, etc., which has been supplied from the environmental sensors, is read again at step 306, and refractive index data necessary for laser wavelength correction is set at step 307. On the basis of the set refractive index data, the laser wavelength is corrected at step 308. At step 309, the reticle and wafer stages are synchronously controlled on the basis of the corrected laser wavelength while the positions of these stages are being measured with the respective laser interferometers, thereby carrying out scanning exposure to transfer a pattern formed on the reticle onto the shot area. Then, 1 is added to the shot area number i at step 310, and a comparison is made at step 311 as to whether or not the number i is greater than the total number n of shot areas. If the number i is not greater than the total number n of shot areas, the process is repeated from step 302. When the number i has become greater than the total number n of shot areas, the exposure process is terminated.
Of the above-described conventional laser wavelength correcting methods, the first method needs to install a special-purpose controller for laser wavelength correction, which causes the cost to increase. The second method suffers from the problem that, during the laser wavelength correcting operation, the reticle and wafer stages are at rest; therefore, the throughput (productivity) reduces correspondingly to the time required for the correcting operation. In this regard, in a one-shot exposure type projection exposure apparatus, once the wafer stage is positioned, exposure is carried out at this position. Therefore, no laser wavelength correction is made again before the exposure, and the reduction of the throughput is only slight. In a scanning exposure type projection exposure apparatus, however, laser wavelength correction is made again before the scanning exposure as at steps 306 to 308 in FIG. 4B. Accordingly, the throughput reduces to a considerable extent particularly in scanning exposure type projection exposure apparatuses.